Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers.
Visible light may include light having many different wavelengths. The apparent color of visible light can be illustrated with reference to a two dimensional chromaticity diagram, such as the 1931 International Conference on Illumination (CIE) Chromaticity Diagram illustrated in FIG. 1, and the 1976 CIE u′v′ Chromaticity Diagram, which is similar to the 1931 Diagram but is modified such that similar distances on the 1976 u′v′ CIE Chromaticity Diagram represent similar perceived differences in color. These diagrams provide useful reference for defining colors as weighted sums of colors.
In the 1976 CIE Chromaticity Diagram, chromaticity values are plotted using scaled u- and v-parameters which take into account differences in human visual perception. That is, the human visual system is more responsive to certain wavelengths than others. For example, the human visual system is more responsive to green light than red light. The 1976 CIE-u′v′ Chromaticity Diagram is scaled such that the mathematical distance from one chromaticity point to another chromaticity point on the diagram is proportional to the difference in color perceived by a human observer between the two chromaticity points. A chromaticity diagram in which the mathematical distance from one chromaticity point to another chromaticity point on the diagram is proportional to the difference in color perceived by a human observer between the two chromaticity points may be referred to as a perceptual chromaticity space. In contrast, in a non-perceptual chromaticity diagram, such as the 1931 CIE Chromaticity Diagram, two colors that are not distinguishably different may be located farther apart on the graph than two colors that are distinguishably different.
As shown in FIG. 1, colors (represented by shades of grey in the figure) on a 1931 CIE Chromaticity Diagram are defined by x and y coordinates (i.e., chromaticity coordinates, or color points) that fall within a generally U-shaped area. Colors on or near the outside of the area are saturated colors composed of light having a single wavelength, or a very small wavelength distribution. Colors on the interior of the area are unsaturated colors that are composed of a mixture of different wavelengths. White light, which can be a mixture of many different wavelengths, is generally found near the middle of the diagram, in the region labeled 100 in FIG. 1. There are many different hues of light that may be considered “white,” as evidenced by the size of the region 100. For example, some “white” light, such as light generated by sodium vapor lighting devices, may appear yellowish in color, while other “white” light, such as light generated by some fluorescent lighting devices, may appear more bluish in color.
Light that generally appears green is plotted in the regions 101, 102 and 103 that are above the white region 100, while light below the white region 100 generally appears pink, purple or magenta. For example, light plotted in regions 104 and 105 of FIG. 1 generally appears magenta (i.e., red-purple or purplish red).
It is further known that a binary combination of light from two different light sources may appear to have a different color than either of the two constituent colors. The color of the combined light may depend on the relative intensities of the two light sources. For example, light emitted by a combination of a blue source and a red source may appear purple or magenta to an observer. Similarly, light emitted by a combination of a blue source and a yellow source may appear white to an observer.
Also illustrated in FIG. 1 is the Planckian locus 106, which corresponds to the location of color points of light emitted by a black-body radiator that is heated to various temperatures. In particular, FIG. 1 includes temperature listings along the Planckian locus. These temperature listings show the color path of light emitted by a black-body radiator that is heated to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish, as the wavelength associated with the peak radiation of the black-body radiator becomes progressively shorter with increased temperature. Illuminants which produce light which is on or near the Planckian locus can thus be described in terms of their correlated color temperature (CCT).
The chromaticity of a particular light source may be referred to as the “color point” of the source. For a white light source, the chromaticity may be referred to as the “white point” of the source. As noted above, the white point of a white light source may fall along the Planckian locus. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source. White light typically has a CCT of between about 2000 K and 10000 K. White light with a CCT of 3000 may appear yellowish in color, while light with a CCT of 8000 K may appear more bluish in color. Color coordinates that lie on or near the Planckian locus at a color temperature between about 2500 K and 8000 K may yield pleasing white light to a human observer.
“White” light also includes light that is near, but not directly on the Planckian locus. A Macadam ellipse can be used on a 1931 CIE Chromaticity Diagram to identify color points that are so closely related that they appear the same, or substantially similar, to a human observer. A Macadam ellipse is a closed region around a center point in a two-dimensional chromaticity space, such as the 1931 CIE Chromaticity Diagram, that encompasses all points that are visually indistinguishable from the center point. A seven-step Macadam ellipse captures points that are indistinguishable to an ordinary observer within seven standard deviations, a ten step Macadam ellipse captures points that are indistinguishable to an ordinary observer within ten standard deviations, and so on. Accordingly, light having a color point that is within about a ten step Macadam ellipse of a point on the Planckian locus may be considered to have a substantially similar color as the point on the Planckian locus.
The ability of a light source to accurately reproduce color in illuminated objects is typically characterized using the color rendering index (CRI). In particular, CRI is a relative measurement of how the color rendering properties of an illumination system compare to those of a reference illuminator, with a reference illuminator for a CCT of less than 5000K being a black-body radiator. For CCT of 5000K and above, the reference illuminator is a spectrum defined by the CIE which is similar to the spectrum of sunlight at the earth's surface. The CRI equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference illuminator. Daylight has the highest CRI (of 100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70-85).
Generally speaking, incandescent bulbs tend to produce more natural-appearing illumination than other types of conventional lighting devices. In particular, incandescent bulbs typically go from a color temperature of about 2700K at full brightness to a color temperature of about 2000 k at 5% brightness and to a color temperature of about 1800K at about 1% brightness. This compares favorably with daylight, which varies from about 6500K at midday to about 2500 k at sunrise and sunset. Research indicates that people tend to prefer warmer color temperatures at low brightness levels and in intimate settings. U.S. Pat. No. 7,213,940 to Van De Ven et al. describes a lighting device using LEDs that can produce warm white light by combining the light from unsaturated yellow (blue-shifted yellow (BSY) LEDs and saturated red LEDs.
In illumination applications, it is often desirable to provide a lighting source that generates a light with a color behavior that approximates the behavior of incandescent lighting. LED-lighting units have been proposed that may be coupled to an AC dimmer circuit and approximate the lighting variation of a conventional incandescent light as the dimmer circuit increases or decreases the brightness of the generated light, as described in U.S. Pat. No. 7,038,399 to Lys et al.
One difficulty with solid state lighting systems including multiple solid state devices is that the manufacturing process for LEDs typically results in variations between individual LEDs. This variation is typically accounted for by binning, or grouping, the LEDs based on brightness, and/or color point, and selecting only LEDs having predetermined characteristics for inclusion in a solid state lighting system. LED lighting devices may utilize one bin of LEDs, or combine matched sets of LEDs from different bins, to achieve repeatable color points for the combined output of the LEDs.
One technique to tune the color point of a lighting fixture is described in commonly assigned United States Patent Publication No. 2009/0160363, the disclosure of which is incorporated herein by reference. The '363 application describes a system in which phosphor converted LEDs and red LEDs are combined to provide white light. The ratio of the various mixed colors of the LEDs is set at the time of manufacture by measuring the output of the light and then adjusting string currents to reach a desired color point. The current levels that achieve the desired color point are then fixed for the particular lighting device. LED lighting systems employing feedback to obtain a desired color point are described in U.S. Publication Nos. 2007/0115662 and 2007/0115228 and the disclosures of which are incorporated herein by reference.